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With eyes bulging and sweat pouring down their faces, submariners crouch fearfully as depth charges explode around them. The sub lurches and shudders, then – in a foaming, noisy climax – water comes pouring in.

This is the popular cinema and TV view of depth charges doing their deadly work against unseen enemies. I find such scenes gripping and unsettling in their intensity – particularly because I hate confined, crowded spaces.

Until 1942 the depth charge was the only weapon that could be used against a submerged submarine. It consisted of a steel drum filled with 200 lbs (90 kilos) of high explosive set to detonate at different depths of water.

In 1939 the standard equipment for small warships was a trap from which charges were rolled over the stern and two mortars, or throwers, which fired them 120 ft on either beam (side of the ship). Soon more traps and throwers were added. Depth charges were dropped in various patterns to give the best chances of success. Eventually heavy weights were fixed to half the charges, causing them to sink faster and explode deeper.

The Battle of the Atlantic gallery at Merseyside Maritime Museum includes a coloured diagram showing a cross section of a Mark VII depth charge. It worked on the principle that water pressure increased with water depth. The depth at which the charge exploded was controlled by an adjustable inlet valve at one end. After filling a bellows chamber, the water drove the detonator against the primer causing it to explode and set off the main charge.

A exploding depth charge could destroy a U-boat 25 ft away and damage one at a distance of 50 ft. Even explosions that didn’t hit their targets could cause trauma, similar to shell shock, among U-boat crews.

Depth charges were used in conjunction with ASDIC, later known as sonar, which had been fitted in many of the Royal Navy’s smaller warships in 1939. It was a secret apparatus for locating submerged submarines using sound waves. The device consisted of an electronic sound transmitter and receiver, housed in a metal dome beneath the ship’s hull, near the bow. The gallery has a life-sized reconstruction of an ASDIC hut on a British destroyer at the start of the war. It features original equipment and a recording of the pinging sounds that bounced back when the sound waves hit a submarine.

After 1942, new weapons such as the forward-throwing Hedgehog and Squid anti-submarine mortars were introduced against U-boats.

A new Maritime Tale by Stephen Guy appears every Saturday in the Liverpool Echo.

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